This application is based on Japanese Patent Application Nos. 2001-103552 filed Apr. 2, 2001 and 2001-104943 filed Apr. 3, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a wavelength converter used for optical communication, optical measurement or display devices, and more particularly to a wavelength converter applicable to optical signal processing that requires high speed, high efficiency and low noise wavelength conversion. In addition, the present invention relates to a wavelength converter as a multi-wavelength light source used for wavelength division multiplexing communication requiring low noise signal light with multiple wavelengths and accurate channel spacing.
2. Description of the Related Art
Conventionally, a wavelength tunable laser, which is implemented by irradiating a crystal or a liquid or gas medium, which possesses second order or third order nonlinearity, with a high power laser beam to convert the laser beam to a wavelength region the laser cannot oscillate, is applicable as a wide range wavelength tunable light source. This technique is generally called an optical wavelength conversion using nonlinear optical media. As for materials of the wavelength conversion media utilizing the secondary nonlinear optical effect, inorganic crystals are applied to many wavelength conversion media at present.
To implement such wavelength conversion, an optical waveguide is often employed to make effective use of the nonlinear optical coefficient of the material. The wavelength converters proposed so far include those utilizing the cross gain modulation, cross phase modulation, and four wave mixing (optical mixing using third-order nonlinear polarization) of optical semiconductors.
In addition, the phase matching is considered as an effective method to be applied to inorganic materials such as KTP and LiNbO3, and techniques are proposed which utilize temperature tuning, angle tuning, and quasi-phase matching in which less cancellation takes place between a nonlinear polarization wave based on a fundamental wave and a propagation high frequency generated.
As for the wavelength conversion utilizing optical semiconductors that are under development at present, they are inapplicable to optical communication or optical measurement that requires high speed and low noise because they have large noise due to their spontaneous emission light, and their speed limit due to carrier lifetime. In addition, although LiNbO3 quasi-phase matching devices are proposed as a high-speed, low-noise wavelength converter, they have drawbacks such as insufficient conversion efficiency, requiring an interaction length of at least 5 cm to achieve preferable conversion efficiency. Furthermore, it has a problem of having polarization sensitivity that the conversion efficiency varies sharply depending on the orientation of the crystal.
Moreover, the domain inversion for the quasi-phase matching must undergo poling using a high voltage, offering a problem of low yields. Besides, since the domain inversion by the poling must be formed such that it makes phase matching with a specified wavelength, the wavelength of the pumping light must be fixed.
As a result, the wavelength converter fabricated has a problem in that it can convert only to a fixed wavelength, and hence cannot convert to a wavelength required. The converting function to a desired wavelength is needed for equipment such as optical switching systems and optical routers, which carry out routing using wavelengths as routing information. In addition, the function is important to circumvent blocking of wavelengths, which can occur when multiple wavelength signals are supplied to a single system.
At present, installation of wavelength division multiplexing (WDM) systems is accelerated to implement large capacity communications. The WDM systems can reduce the cost of a system by transmitting multiple signals with different wavelengths through a single optical fiber. Therefore, it can increase the transmission capacity without installing a new fiber.
Although the method has an advantage in the fiber installation cost, it has a problem of requiring many light sources with high wavelength accuracy to achieve high density. Up to now, a method is used which selects semiconductor lasers that precisely fit to the wavelengths of the signal light, and disposes them by the number required. This method, however, has a problem of increasing cost because of the selection of lasers suitable for the wavelengths.
Alternatively, a method using a semiconductor mode-locking laser or fiber ring laser is also proposed. In addition, a spectral slice light source is proposed which slices supercontinuum (SC) light that is generated by the short-pulse light source and nonlinear optical fiber by an arrayed waveguide grating demultiplexer. However, since it requires a long nonlinear fiber to generate the SC light, it has a problem of making it difficult to reduce its size.
The present invention is implemented considering the foregoing problems. Therefore, an object of the present invention is to provide a high efficiency, low noise wavelength converter that can be implemented without the high voltage poling of a crystal, and that can carry out switching and modulation of converted light by using electric field.
Another object of the present invention is to provide a wavelength converter functioning as a multi-wavelength light source capable of controlling a wavelength band or the number of wavelengths by selecting electrodes to which electric fields are applied.
To accomplish the objects, according to the present invention, there is provided a wavelength converter for producing converted light with a wavelength corresponding to an energy difference between signal light and pumping light with a wavelength different from that of the signal light, by launching the signal light and the pumping light into a crystal material simultaneously, wherein the crystal material consists of a crystal composed of at least one of KTa1-xNbxO3 and K1-yLiyTa1-xNbxO3.
In addition, to accomplish the objects, according to a present invention, there is provided a wavelength converter operating as a multi-wavelength light source including a planar optical waveguide comprising: a core with a high refractive index composed of a crystal material with a composition of at least one of KTa1-xNbxO3 and K1-yLiyTa1-xNbxO3; a cladding surrounding the core; an electrode that is formed on a surface of the optical waveguide and has a fixed electrode period; signal light generating means for generating signal light with at least one wavelength; and pumping light generating means for generating pumping light with a wavelength different from that of the signal light output from the signal light generating means, wherein the signal light and the pumping light are launched simultaneously into the optical waveguide to generate signal light with at least one wavelength.
Thus, the present invention is characterized in that it utilizes the crystal with the composition of KTa1-xNbxO3 and/or K1-yLiyTa1-xNbxO3 as a medium for achieving the wavelength conversion. These KTN and KLTN crystals are a cubic system with centrosymmetry in an applied temperature range. Although they have no second order nonlinear effect, they are characterized by exhibiting secondary nonlinear effect in response to an electric field applied. Therefore, it is possible to implement the multiple wavelength generation based on the differential frequency generation by applying the electric field to the electrode with the period that makes phase matching with the signal light and pumping light.
The efficiency of the nonlinear optical effect increases in proportion to the electric field applied, and offers a twice or more efficiency as compared with the conventional LiNbO3 nonlinear optical crystal within a range of a practical electric field to be applied. Accordingly, it can implement the wavelength conversion with four times or more efficiency using the same interaction length as the conventional LN wavelength converter, or with the same efficiency using less than half the interaction length. In addition, when the electric field is removed, the KTN and KLTN crystals are simply a transparent medium without causing any changes in the signal light. Thus, they can achieve such a function as turning the converted light on and off by switching the electric field on and off. In addition, the converted light can be modulated by modulating the electric field applied.
In addition, as for the conventional wavelength converter, since the LN crystal is a trigonal system, the c axis must be aligned to the polarization of the incident light to obtain the maximum nonlinear effect, and the quasi-phase matching is achieved by inverting the spontaneous polarization in the c axis direction. Therefore, in the differential frequency generation by the LN wavelength converter, the polarization direction of convertible light is limited by the direction of the domain inversion produced, making it impossible to achieve high conversion efficiency in the other polarization. In contrast, the KTN and KLTN used in the present invention are an isotropic crystal with exhibiting a nonlinear characteristic in the direction of the applied electric field. Thus, they have an advantage of being able to implement the polarization insensitive wavelength converter easily with such a structure as including two electrodes perpendicular to each other to which the electric fields are applied.
Furthermore, the wavelength converter in accordance with the present invention has an advantage of being able to obviate the need for the high voltage poling of the crystal which is required by the conventional LN wavelength converter, and to implement the quasi-phase matching easily by forming the electrode. This is because forming several type of electrodes with different periods on the surface of the crystal makes it possible to select the wavelength of the pumping light in accordance with the period, thereby being able to provide the wavelength converter with those functions. Furthermore, since the principle of the wavelength conversion in accordance with the present invention is based on the differential frequency generation, which is a parametric process, it offers an advantage of high speed beyond THz and noise free characteristic. Thus, it can implement the performance that no wavelength conversion using the optical semiconductors can achieve. Besides, since the converted light is generated by the interaction between the signal light and pumping light, it is shaped up into a pulse train consisting of short-width pulses. Accordingly, when the pumping light consists of a short-width pulse train such as that of a fiber-ring laser, even if the signal light is generated by a broad light source such as a semiconductor laser generating light including jitters, the wavelength converter in accordance with the present invention can generate high quality light.
Furthermore, differential frequencies, the number of which corresponds to the number of the electrodes, can be obtained by disposing the electrodes with different periods in the direction of the waveguide, by launching the pumping light that phase matches with the periods, and by applying the electric fields to all the electrodes. When the initial incident signal light has multiple wavelengths, the number of wavelengths the device can produce is equal to nxc3x972m, where n is the number of wavelengths of the initial incident signal light, and m is the number of electrodes. For example, when the number of the wavelengths of the initial incident light is 10, and the number of the electrodes is four, it can generate 160 waves.
In addition, since the channel spacing of the signal light generated by this method is determined by energy difference between the channel spacing of the initial incident signal light and the wavelength corresponding to half the energy of the pumping light, the wavelength converter in accordance with the present invention can generate the light with a uniform channel spacing precisely matching the ITU-T grid.
Furthermore, it has an advantage of being able to offer high speed beyond THz and noise free characteristic in principle. In addition, it operates as a wavelength tunable light source by sequentially applying the electric field via the electrodes that have quasi-phase matching with different wavelengths. The light source can also operate as a variable wavelength light source incorporating a modulator, because it can generate a modulated signal by modulating the electric field by some other method.
Although the embodiments below utilize a rectangular buried waveguide, similar characteristics can be achieved by a diffusion waveguide fabricated using ion diffusion.
Thus, according to the present invention, the crystal material consists of a crystal composed of KTa1-xNbxO3 and/or K1-yLiyTa1-xNbxO3 in the wavelength converter that produces converted light with a wavelength corresponding to an energy difference between the signal light and pumping light with a wavelength different from that of the signal light by launching the signal light and the pumping light into the crystal material simultaneously. As a result, the present invention can implement the high efficiency, low noise wavelength conversion without performing the high voltage poling of the crystal which is essential for the conventional wavelength converter. In addition, it can achieve the switching and modulation of the converted light by the electric field.
Moreover, it can achieve the polarization insensitive wavelength conversion, which is impossible for the conventional converter. This enables the optical signal processing indispensable for the optical routing applied to the optical communication field, thereby implementing a router with simple configuration at low cost. The wavelength conversion is free from noise, and causes no signal degradation even through the wavelength conversion is repeated by a number of stages. Accordingly it is applicable to a router that repeats the signal processing many times. In addition, in the optical measurement field, it can demultiplex a ultra-fast optical signal at high efficiency, offering an advantage of being able to fabricate ultra-fast optical signal measuring instruments with a simple configuration.
As for other applications, using the wavelength converter in accordance with the present invention can implement high wavelength conversion efficiency that cannot be achieved by conventional devices, and the second harmonic generation by the converter makes it possible to use it as a blue color emitted laser light source.
Furthermore, according to the present invention, the wavelength converter includes the electrode that is formed on a surface of the optical waveguide and has a fixed electrode period; a signal light generating means for generating signal light with at least one wavelength; and a pumping light generating means for generating pumping light with a wavelength different from that of the signal light output from the signal light generating means, wherein the signal light and the pumping light are launched simultaneously into the optical waveguide to generate signal light with at least one wavelength. Thus, it can implement a multi-wavelength light source, which cannot be realized by the conventional technique, on a single chip. In addition, it can control the number of wavelengths and wavelength band by selecting the electrodes to which the electric field is applied. Furthermore, it offers an advantage of being able to generate the short-pulse signal light with ease. Thus, the present invention can implement the multi-wavelength light source applied to the wavelength division multiplexing communication with a simple and inexpensive configuration.
As described above, the KTN crystal and KLTN crystal used in the present invention assume that they are used as a cubic system. However, the ferroelectric phase transition temperature from the cubic to tetragonal system is controllable in a range of xe2x88x92250xc2x0 C.-400xc2x0 C. by varying composition of the Nb and Ta. In this case, by using a crystal with the phase transition temperature above the room temperature, and by cooling it below the phase transition temperature with applying the electric field via the electrode, the spontaneous polarization occurs in the direction of the electric field applied, and is fixed. A wavelength converter requiring no application of the electric field can be configured by controlling the phase transition temperature. The polarization structure thus configured can be eliminated by elevating its temperature beyond the phase transition temperature.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.